This invention relates generally to equipment for the freezing of foods and other substances and, more particularly, to a wheel or drum type freezer and method of operating same to achieve very low temperatures and high heat transfer rates.
The use of drum or wheel type freezers for the rapid freezing of foods is illustrated in U.S. Pat. No. 4,192,899 to Roth. The freezer disclosed in that patent generally comprises a revolving drum having an outer skin of a smooth metallic material which has a high heat transfer capacity A refrigerant under high pressure is circulated through an interior area of the drum to cool the outer skin and the food product placed thereon The food product is in the form of a sheet of beef which is frozen and subsequent cut into patties
Freezers of the type shown in the '899 patent are designed to provide at least partial freezing of the food product as it is carried along on the outer skin of the drum. Such freezers require only a limited amount of floor space in a commercial operation and also reduce the time required to freeze processed food products. This accelerated freezing is particularly desirable in that it reduces the amount of time that bacterial growth may occur while processed food is awaiting freezing.
The heat transfer rates that can be achieved by conventional drum type freezers for food products is determined in part by the circulation pattern of the refrigerant within the drum. Typically, the refrigerant is circulated through a peripheral chamber in contact with the inner surface of the drum's outer skin. Even at high refrigerant flow rates and pressures, a generally stationary boundary layer of refrigerant remains positioned adjacent the inner surface of the outer skin. This boundary layer of refrigerant has been warmed by the heat transfer across the outer skin and thus impedes heat transfer between the circulating refrigerant and the drum surface.
The use of a refrigerant circulation system for an ice making machine is disclosed in U.S. Pat. No. 2,900,804 to Rising. The ice machine shown therein utilizes a plurality of apertures in a wall of an annular chamber to direct the high pressure refrigerant radially outward against the inner circumferential wall of the drum. The liquid then travels across the length of the drum before it is directed to a center shaft for removal from the drum. While this type of a system facilitates heat transfer across the outer skin of the drum, the large spacing between the apertures and the circumferential wall causes the newly charged refrigerant to mix with warmer refrigerant that is flowing toward the exit channel. The temperature of the refrigerant contacting the wall is thus elevated and provides less efficient cooling of the drum surface. In addition, the migration of the spent refrigerant toward the exit channel at the end of the drum causes significant temperature variations along the drum surface. While this mixing of the refrigerant and the resulting temperature variations do not present a problem when freezing a substance such as water having a high freezing point and low incidence of bacterial growth, this concept is poorly suited for use to freeze food and other types of substances requiring much lower and more uniform freezing temperatures.
Conventional wheel type freezers such as disclosed in U.S. Pat. Nos. 4,192,899 and 2,900,804 discussed above operate under high pressures in order to maintain the heat transfer fluid in a liquid state. Such high pressures require that all fittings and components be of sufficient strength to withstand the pressure, thus adding significantly to the cost of those freezers The freezers must also undergo extensive testing and certification procedures in order to comply with governmental safety regulations because of the extensive injury which could occur should a fitting rupture and spray the low temperature fluid on nearby personnel or equipment.